The Louisiana Democrat chartered a private plane last September to travel from New Orleans to Shreveport, where she attended an official event. She then traveled on to Dallas, where she attended a fundraiser.

Under federal law and Senate rules, the cost of a trip that includes official and campaign stops must often be prorated between Senate and campaign accounts. However, if the campaign activity is "incidental" to the official trip, the expense doesn't need to be split up. In this case, Landrieu's Senate office picked up the whole cost of the trip.

Asked Wednesday about the trip, Landrieu's Senate spokesman, Matthew Lehner, said in a statement:

"Sen. Landrieu was invited to an official evening event in Shreveport. She was traveling back to D.C. through Dallas, and a fundraiser in Dallas was added to her schedule," he said. "Out of an abundance of caution in case there was a cost allocation error connected to this flight, the Senate will be reimbursed for the (Shreveport to Dallas) flight."

A Senate aide said that because Landrieu was already going to be in Dallas, the office considered the fundraiser incidental and not an expense that had to be partially covered by the campaign.

The reimbursement comes just one day after CNN learned that the vulnerable Louisiana Democrat used government money last November to charter a private plane to travel to a campaign fundraiser, in violation of federal law.

Landrieu spent more than $3,200 in taxpayer money to fly 400 miles round trip from New Orleans to Lake Charles, Louisiana, where she attended a $40-per person fundraising lunch with hundreds of women, according to Senate records and Landrieu campaign information. It is illegal to spend government money campaigning.

Landrieu's campaign spokesman, Fabien Levy, said in a statement Monday that the charter company mistakenly billed Landrieu's Senate office instead of her re-election campaign. Levy said the campaign noticed the error a few weeks ago and asked the company to refund the Senate office and bill the campaign, which the company did. Levy said Landrieu's re-election campaign paid for the flight August 4, almost nine months after the November 8 trip.

"We take our finances very seriously and are glad we caught the vendor's mistake and were able to rectify the matter as soon as possible," Levy said.

The campaign didn't notice the mistake until July 29. Two days later, on July 31, USA Today published a story showing that Landrieu was one of the Senate's most frequent fliers, spending $47,000 last year on taxpayer-funded charter flights. Levy told CNN that media scrutiny of Senate office expense reports spurred the campaign to review its travel records.